1. Field of the Invention
The present invention relates to a novel particulate porous carbon material which comprises a carbon phase C and a pore phase, and to the use of such materials in lithium cells, especially lithium-sulfur cells. The invention also relates to a process for producing such carbon materials and to composite materials comprising elemental sulfur and at least one inventive particulate porous carbon material.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In an increasingly mobile society, mobile electrical devices are playing an ever greater role. For many years, batteries, especially rechargeable batteries (called secondary batteries or accumulators), have therefore been finding use in virtually all areas of life. There is now a complex profile of demands on secondary batteries with regard to the electrical and mechanical properties thereof. For instance, the electronics industry is demanding new, small, lightweight secondary cells or batteries with high capacity and high cycling stability to achieve a long lifetime. In addition, the thermal sensitivity and the self-discharge rate should be low in order to ensure high reliability and efficiency. At the same time, a high level of safety in the course of use is required. Lithium secondary batteries with these properties are especially also of interest for the automotive sector, and can be used, for example, in the future as energy stores in electrically operated vehicles or hybrid vehicles. In addition, there is a requirement here for batteries which have advantageous electrokinetic properties in order to be able to achieve high current densities. In the development of novel battery systems, there is also a special interest in being able to produce rechargeable batteries in an inexpensive manner.
Environmental aspects are also playing a growing role in the development of new battery systems.
The cathode of a modern high-energy lithium battery now comprises, as an electroactive material, typically lithium-transition metal oxides or mixed oxides of the Spinel type, for example LiCoO2, LiNiO2, LiNi1-x-yCoxMyO2 (0<x<1, y<1, M e.g. Al or Mn) or LiMn2O4, or lithium iron phosphates, for example. For the construction of the anode of a modern lithium battery, the use of lithium-graphite intercalation compounds has been proven in the last few years (Journal Electrochem. Soc. 1990, 2009). In addition, as anode materials, lithium-silicon intercalation compounds, lithium alloys and lithium titanate have been examined (see K. E. Aifantis, “Next generation anodes for secondary Li-ion batteries” in High Energy Density Li-Batteries, Wiley-VCH, 2010, p. 129-162). The two electrodes are combined with one another in a lithium battery using a liquid or else solid electrolyte. In the (re)charging of a lithium battery, the cathode material is oxidized (for example according to the following equation: LiCoO2→nLi++Li(1-n)Co)2+ne−). This releases the lithium from the cathode material and it migrates in the form of lithium ions to the anode, where the lithium ions are bound with reduction of the anode material, and in the case of graphite intercalated as lithium ions with reduction of the graphite. In this case, the lithium occupies the interlayer sites in the graphite structure. In the course of discharging of the battery, the lithium bound within the anode is removed from the anode in the form of lithium ions, and oxidation of the anode material takes place. The lithium ions migrate through the electrolytes to the cathode and are bound therein with reduction of the cathode material. Both in the course of discharging of the battery and in the course of recharging of the battery, the lithium ions migrate through the separator.
A central problem in the case of batteries in general, but also in the case of lithium batteries, is the limited capacity density or energy density thereof. In recent times, there has therefore been increasing emphasis on lithium-sulfur cells, i.e. lithium cells whose cathode material comprises sulfur in elemental or in polymer-bound form. In the lithium-sulfur cell, polysulfide anions are formed at the cathode in the course of discharge, and are discharged at the anode. The chemical reaction and the cathode can be represented in simplified form as follows: 2Li++Sx+2e−→L+2Sx2−, with a decreasing number of sulfur atoms in the polysulfide anions formed as the discharging operation progresses. With a theoretical energy density of nearly 2600 Wh/kg (assuming a full reaction to give Li2S—see Journal Material Chemistry, 2010, 20, 2821-2826), lithium-sulfur cells have a theoretical energy density more than 4 times higher than conventional lithium ion cells with cathode materials based on lithium-transition metal oxides.
In the case of lithium-sulfur cells, however, the relatively rapid loss of capacity with repeated charging and the associated limit in the lifetime of the cell have been found to be problematic to date. This capacity loss is attributable to the loss of sulfur from the cathode material, caused by the formation of the polysulfide anions which are soluble in the electrolyte. Since elemental sulfur itself is an insulator, greater amounts of conductive additives such as conductive blacks or metal particles have to be used in sulfur-based cathode materials. Owing to the migration of sulfides, however, insulating sulfur layers can be formed on the surface of the anode or in the region of the separator, as a result of which the cell resistance and the impedance increase.
There are in principle several approaches to counteracting the sulfur migration and the associated loss of sulfur in the cathode material.                Binding of the sulfur atoms to a carbonaceous polymer skeleton such as polycarbon sulfide (see, for example, U.S. Pat. No. 4,833,048, WO 96/41387, U.S. Pat. Nos. 6,117,590, 6,309,778). A disadvantage is found to be the complex preparation and difficulty of handling of the polymers;        use of transition metal-doped molybdenum sulfides, tungsten sulfides or titanium sulfides (see, for example, U.S. Pat. Nos. 6,300,009, 6,319,633 or 6,376,127). In this case, a lower specific capacity of the materials is accepted since the transition metals make up a comparatively high proportion of the cathode material;        use of composite materials composed of sulfur or sulfur-containing electroactive materials and adsorbents with affinity for polysulfide anions, such as porous carbon (aerogels), silica gels, aluminum oxides such as boehmite and pseudo-boehmite (see, for example, U.S. Pat. No. 5,162,175, WO 99/33131, WO 2009/114314).        
B. Zhang et al, Energy Environ. Sci. 2010, 3, 1531-1537 describe a carbon/sulfur material as a cathode material for Li-sulfur cells, in which sulfur is intercalated in micropores of a microporous carbon material. The composite material, however, exhibits satisfactory discharge capacities only up to a loading with 42% sulfur.
In summary, it can be stated that the carbon/sulfur-based cathode materials known to date from the prior art are unsatisfactory with regard to the charging/discharging kinetics, and/or the cycling stability, for example decrease in the capacity, and/or high or increasing impedance after several charge/discharge cycles. The composite materials comprising a particulate semimetal or metal phase and one or more carbon phases, which have been proposed in recent times to solve these problems, are capable of solving these problems only partially, and the quality of such composite materials generally cannot be achieved in a reproducible manner. In addition, the production thereof is generally so complex that economic use is impossible.